As documented by Wikipedia, vocalist Robert Plant said in a bootleg recording from a 1970 show:
This is a thing called ‘Bron-Yr-Aur’. This is a name of the little cottage in the mountains of Snowdonia in Wales, and ‘Bron-Yr-Aur’ is the Welsh equivalent of the phrase ‘Golden Breast’. This is so because of its position every morning as the sun rises and it’s a really remarkable place. And so after staying there for a while and deciding it was time to leave for various reasons, we couldn’t really just leave it and forget about it. You’ve probably all been to a place like that, only we can tell you about it and you can’t tell us.